The impending showdown between Michael Phelps and Ryan Lochte is one of several great rivalries to watch in London this summer

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The impending showdown between Michael Phelps and Ryan Lochte is one of several great rivalries to watch.

One of the Olympic’s greatest rivalries will culminate later in London with a final showdown between American swimmers Michael Phelps and Ryan Lochte.

Lochte crushed his teammate in their first London face-off in the 400m individual medley to win America's first gold of the 2012 Games. Phelps came in fourth more than four seconds behind.

They will go head-to-head once more, on Aug. 2 in the 200m individual medley.

Here's a primer on their rivalry and four other great duels we're watching in London:

Best Olympic Bodies

Men's Swimming: Phelps vs. Lochte

This competition dates back to the 2004 Games in Athens, when Phelps won six gold medals, beating Lochte in the 200m individual medley.

Four years later, Phelps won a record eight gold medals while Lochte won two, including a world-record breaking performance in the 200m backstroke that went largely overlooked.

Athletes to Watch: London 2012

After Beijing, the power balance began to shift. Phelps lost some of his drive, and Lochte seized the opportunity. “After 2008, I said enough is enough,” he told "Outside" magazine.

In the 2009 world championships, Phelps won five golds, Lochte four. The following year, at the Pan Pacific Championships, Lochte won five golds, Phelps four. And at the 2011 world championships, Lochte beat Phelps in both of their head-to-head matchups, taking Phelps’ title as the world’s best swimmer.

This pairing could be the most underrated of the Games. Felix, an American, and Campbell Brown, a Jamaican, have been battling each other in the 200m since 2004, trading positions as the world’s top and second-ranked sprinters. Felix has won three world championships, but says she’d give them up for Campbell Brown’s two Olympic gold medals.

Despite running neck-and-neck for so many years, their duels haven’t gotten as much media attention as Tyson Gay and Usain Bolt (see below), which Campbell Brown blames on latent sexism. "We're not treated equally as our male counterparts," she told the Guardian.

Barring injury or another unexpected event, it’s practically assumed that these two women will meet in the 100m finals in London and finish 1-2. The suspense is over who will edge out the other.

Women’s gymnastics: United States vs. China

The Americans and Chinese dueled furiously four years ago in Beijing, with the Chinese taking all-around team gold and the Americans taking gold and silver in the all-around individual competition. Those Games were marred by unproven allegations that the Chinese had used underage athletes.

China slipped at subsequent world championships, repeatedly falling short of the United States; at last year’s competition, the Americans dominated, while China came in third. China’s chances at defending their all-around team gold suffered this spring, when its leading star, Cheng Fei, got hurt and had to withdraw. That makes the U.S. team the clear favorite in London.

If the Chinese don’t step up to challenge the Americans, then the Russians will, reigniting what was once the sport’s biggest rivalry.

Men’s gymnastics: China vs. Japan

No country has been more dominant in the last decade than China, which won the all-around team gold in 2008. Japan took silver that year (and the Americans bronze).

Today, however, Japan is the world’s ascendant gymnastic power, led by Kohei Uchimura, the reigning all-around Olympic champion and still the world’s top male gymnast.

The Americans believe they’re contenders, and stole the show July 28 in the qualifying round. Britain scored a surprise second, while Japan finished in third and China stumbled to fourth place.

Still all scores reset to zero for team finals.

Track and field: Usain Bolt vs. Tyson Gay

Bolt, a Jamaican, is the world’s fastest runner and the reigning world record holder in the 100m and 200m. Gay, an American, is the second fastest, and the last person to beat Bolt, in 2010.

Bolt says he lost to Gay because of a back injury, and insists that he won’t lose in London.

Gay, meanwhile, is struggling to recover from a series of injuries suffered since his disappointing performance in the 2008 Olympics, in which he failed to win a medal. He has a tough field of countrymen to surpass at the Olympic trials before even getting a shot at Bolt.

Gay said he’s not thinking about his Bolt as much as earning an Olympic medal. “It’s more about me reaching my goals,” he said.